The industrial robot has an assembly of a plurality of movable robot elements which are articulated in functional sequence by joint structures. The joint structures are operated to provide the motions of the movable robot elements, and thus enable an end effector provided on the extremity of the robot unit to achieve the desired robot motions. Generally, a joint structure is provided in the base unit of a robot assembly, to turn movable elements provided on the extremity of the robot body or the vertical shaft, including a robot arm, a robot wrist and an end effector, about a vertical axis by turning the robot body or the upright shaft, and usually, the joint structure provided in the base unit is provided with a gear transmission mechanism or a reduction gear. When the joint structure is provided with a gear transmission mechanism, however, the working accuracy of the end effector provided on the extremity of the robot assembly is directly affected by the transmission accuracy of the gears of the gear transmission mechanism. For example, a backlash always remains between the teeth of the mating gears of the gear transmission mechanism, and accordingly, an uncontrollable zone appears in the motion of the movable robot element driven through the gear transmission mechanism by a drive source when the gears of the gear transmission mechanism are reversed, and thus precise robot motions cannot be obtained. The applicant of the present patent application previously proposed a backlash eliminator for a reduction gear applicable to the joint structure of the base unit of a robot assembly, in Japanese Unexamined (Kokai) Patent Publication No. 63-23064, to solve the problems attributable to this backlash. This previously proposed backlash eliminator comprises two pulleys provided one above the other on the output end of a drive motor and able to turn relative to each other, a first pinion engaging a driven gear and driven through a transmission belt (toothed belt) by one of the pulleys, and second pinion engaging the same driven gear and driven through a transmission belt by the other pulley whereby backlash is eliminated by the relative rotation of the two pulleys.
A gear transmission mechanism or a reduction gear mechanism incorporating such a backlash eliminator has two power transmission routes, due to the employment of the two pulleys, and the power transmission belts are usually properly tensioned by tension devices, to secure a precise power transmission. Therefore, when the power transmission belts are improperly tensioned, or the power transmission belts are not periodically changed, the power transmission belts are readily broken; particularly when the backlash eliminator is included in the power transmission system of an industrial robot, in which the power transmission system is required to transmit a comparatively large power. If one of the two transmission belts is broken the load on the other transmission belt is sharply increased, and the other transmission belt, which also has been worn to a significant extent by that time, may break shortly after the former transmission belt is broken. Accordingly, since the drive side of the power transmission system and the driven gear are interlocked only by the two transmission belts, the movable robot elements operatively connected to the driven gear may become free and uncontrollable when the transmission belts are broken.